CBSE - Sources of Energy

A good source of energy fulfils the following criteria:

(I)It produces a lot of heat per unit mass.

(II) It does a huge amount of work per unit mass.

(III) It is easily accessible.

(IV) It is easy to store and transport.

(V)It is economical.

(VI)It produces less amount of smoke.

A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.

Natural gas can be used for heating and cooking food because it is a clean source of energy. It does not produce huge amount of smoke on burning. Although it is highly inflammable, it is easy to use, transport, and it produces a huge amount of heat on burning.

The disadvantages of fossil fuels are:
→ Burning of coal and petroleum produces a lot of pollutants causing air pollution.
→ Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause acid rain, which affects the soil fertility and potable water.
→ Burning of fossil fuels produce gases such as carbon dioxide that causes global warming.

Fossil fuels which are traditionally used by human beings everywhere as an energy sources are non-renewable sources of energy. These sources of energy are limited and will disappear after some time. They are being consumed at a large rate. Therefore, we should conserve the energy sources. Hence, we should look for alternate sources of energy.

Earlier, the windmills were used to harness wind energy to do mechanical work such as lifting/drawing water from a well. Today, windmills are used to generate electricity.In windmills, the kinetic energy of wind is harnessed and converted into electricity.
Water energy which was used for transportation before is now a good source to generate electricity. Dams has been constructed on river for generating electricity. Waterfalls were used as a source of potential energy which was converted to electricity with the help of turbines.

A concave mirror is used in a solar cooker as it uses heat of the sunlight to cook food. The mirror focuses all the incident sunlight at a point. The temperature at that point increases, thereby cooking and heating the food placed at that point.

The forms of energy that can be obtained from the ocean are tidal energy, wave energy, and ocean thermal energy. There are several limitations in order to harness these energies.

(i) Tidal energy depends on the relative positioning of the Earth, moon, and the Sun.

(ii) High dams are required to be built to convert tidal energy into electricity.

(iii) Very strong waves are required to obtain electricity from wave energy.

(iv) To harness ocean thermal energy efficiently, the difference in the temperature of surface water (hot) and the water at depth (cold) must be 20ºC or more.

Geothermal power plants use heat of the Earth to generate electricity. This heat energy of the Earth is known as geothermal energy.

The advantages of nuclear energy areas follows:

(a) Large amount of energy is produced per unit mass.

(b) It does not produce smoke. It is a clean energy.

(c) Fission of one atom of uranium produces 10 million times the energy released by burning of one atom of carbon.

(d) Fusion of four hydrogen atoms produces huge amount of energy approximately equal to 27 MeV.

Two renewable sources of energy are as follows:

(a) Sun: The energy derived from the Sun is known as solar energy. Solar energy is produced by the fusion of hydrogen into helium, fusion of helium into other heavy elements, and so on. A large amount of hydrogen and helium is present in the Sun. Therefore, solar energy can replenish on its own. The Sun has 5 billion years more to burn. Hence, solar energy is a renewable source of energy.

(b) Wind: Wind energy is derived from air blowing with high speed. Wind energy is harnessed by windmills in order to generate electricity. Air blows because of uneven heating of the Earth. Since the heating of the Earth will continue forever, wind energy will also be available forever.

Two exhaustible energy sources are as follows:

(a) Coal: It is produced from dead remains of plants and animals that remain buried under the earth's crust for millions of years. It takes millions of years to produce coal. Industrialization has increased the demand of coal. However, coal cannot replenish within a short period of time. Hence, it is a non-renewable or exhaustible source of energy.

(b) Wood: It is obtained from forests. Deforestation at a faster rate has caused a reduction in the number of forests on the Earth. It takes hundreds of years to grow a forest. If deforestation is continued at this rate, then there would be no wood left on the Earth. Hence, wood is an exhaustible source of energy.

A solar water heater cannot be used to get hot water on

(a) a sunny day (b) a cloudy day

(c) a hot day (d) a windy day

Ans :

(b) A solar water heater uses solar energy to heat water. It requires bright and intense sunlight to function properly. On a cloudy day, the sunlight reflects back in the sky from the clouds and is unable to reach the ground. Therefore, solar energy is not available for the solar heater to work properly. Hence, solar water heater does not function on a cloudy day.

Which of the following is not an example of a bio-mass energy source?

(a) wood

(b) gobar gas

(c) nuclear energy

(d) coal

Answer :

(c) Bio-mass is a source of energy that is obtained from plant materials and animal wastes. Nuclear energy is released during nuclear fission and fusion. In nuclear fission, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear fusion reaction, lighter nuclei are fused together to form a relatively heavier nuclei. This reaction produces tremendous amount of energy. Hence, nuclear energy is not an example of bio-mass energy source.

Wood is a plant material, gobargas is formed from animal dung, and coal is a fossil fuel obtained from the buried remains of plants and animals. Hence, these are bio-mass products. 

Most of the sources of energy we use represent stored solar energy. Which of the following is not ultimately derived from the Sun's energy?

(a) Geothermal energy

(b) Wind energy

(c) Nuclear energy

(d) Bio-mass

 
Answer :

(c) Nuclear energy is released during nuclear fission and fusion. In nuclear fission, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear fusion reaction, lighter nuclei are fused together to form a relatively heavier nuclei. The energy required to fuse the lighter nuclei is provided by fission reactions. This reaction produces tremendous amount of energy. These reactions can be carried out in the absence or presence of sunlight. There is no effect of sunlight on these reactions. Hence, nuclear energy is not ultimately derived from Sun's energy.

Geothermal energy, wind energy, and bio-mass are all ultimately derived from solar energy.

Geothermal energy is stored deep inside the earth's crust in the form of heat energy. The heating is caused by the absorption of atmospheric and oceanic heat. It is the sunlight that heats the atmosphere and oceans.

Wind energy is harnessed from the blowing of winds. The uneven heating of the earth's surface by the Sun causes wind.

Bio-mass is derived from dead plants and animal wastes. Chemical changes occur in these dead plants and animal wastes in the presence of water and sunlight. Hence, bio-mass is indirectly related to sunlight.

Fossil fuels are energy sources, such as coal and petroleum, obtained from underneath the Earth’s crust. They are directly available to human beings for use. Hence, fossil fuels are the direct source of energy. These are limited in amount. These are non-renewable sources of energy because these cannot be replenished in nature. Fossil fuels take millions of years for their formation. If the present fossil fuel of the Earth gets exhausted, its formation will take several years. Fossil fuels are also very costly.
On the other hand, solar energy is a renewable and direct source of energy. The Sun has been shining for several years and will do so for the next five billion years. Solar energy is available free of cost to all in unlimited amount. It replenishes in the Sun itself.

Bio-mass and hydro-electricity both are renewable sources of energy. Bio-mass is derived from dead plants and animal wastes. Hence, it is naturally replenished. It is the result of natural processes. Wood, gobargas, etc. are some of the examples of bio-mass.
Hydro-electricity, on the other hand, is obtained from the potential energy stored in water at a height. Energy from it can be produced again and again. It is harnessed from water and obtained from mechanical processes.

(a) A windmill requires wind of speed more than 15 km/h to generate electricity from wind energy also large numbers of windmills are required to get feasible output which covers a large area.

(b) Very strong ocean waves are required in order to extract energy from waves.

(c) Very high tides are required in order to extract energy from tides. Also, occurrence of tides depends on the relative positions of the Sun, moon, and the Earth.

(a) The source of energy that replenishes in nature is known as renewable source of energy. Sun, wind, moving water, bio-mass, etc. are some of the examples of renewable sources of energy.
The source of energy that does not replenish in nature is known as non-renewable source of energy. Coal, petroleum, natural gas, etc. are some of the examples of non-renewable sources of energy.

(b) Exhaustible sources are those sources of energy, which will deplete and exhaust after a few hundred years. Coal, petroleum, etc. are the exhaustible sources of energy.
Inexhaustible resources of energy are those sources, which will not exhaust in future. These are unlimited. Bio-mass is one of the inexhaustible sources of energy.

Yes. The options given in (a) and (b) are the same.

An ideal source of energy must be:
→ Economical
→ Easily accessible
→ Smoke/pollution free
→ Easy to store and transport
→ Able to produce huge amount of heat and energy on burning

Solar cooker uses Sun’s energy to heat and cook food. It is inexhaustible and clean renewable source of energy. It is free for all and available in unlimited amount. Hence, operating a solar cooker is not expensive.
Disadvantage of a solar cooker is that it is very expensive. It does not work without sunlight. Hence, on cloudy day, it becomes useless.
The places where the days are too short or places with cloud covers round the year, have limited utility for solar cooker.

Industrialization increases the demand for energy. Fossil fuels are easily accessible sources of energy that fulfil this demand. The increased use of fossil fuels has a harsh effect on the environment. Too much exploitation of fossil fuels increases the level of green house gas content in the atmosphere, resulting in global warming and a rise in the sea level.
It is not possible to completely reduce the consumption of fossil fuels. However, some measures can be taken such as using electrical appliances wisely and not wasting electricity. Unnecessary usage of water should be avoided. Public transport system with mass transit must be adopted on a large scale. These small steps may help in reducing the consumption of natural resources and conserving them.

1. Which of the following is a non-renewable source of energy?
(a) Wood
(b) Sun
(c) Fossil fuels
(d) Wind
Ans. (c) Fossil fuels

Explanation: It takes millions of Years of the formation of fossil fuels. So, they cannot be renewed in foreseeable future.
2. Acid rain happens because
(a) sun leads to heating of upper layer of atmosphere
(b) burning of fossil fuels release oxides of carbon, nitrogen and sulphur in the atmosphere
(c) electrical charges are produced due to friction amongst clouds
(d) earth atmosphere contains acids
Ans. (b) burning of fossil fuels release oxides of carbon, nitrogen and Sulphur in the atmosphere
Explanation: Oxides of carbon, nitrogen and sulphur mix with rain water to form acids. This causes acid rain.

3. Fuel used in thermal power plants is
(a) water
(b) uranium
(c) biomass
(d) fossil fuels
Ans. (d) fossil fuels
Explanation: Coal is the major fuel used in thermal power plants. Additionally, natural gas and petroleum gas are also used.

4. In a hydro power plant
(a) Potential energy possessed by stored water is converted into electricity
(b) Kinetic energy possessed by stored water is converted into potential energy
(c) Electricity is extracted from water
(d) Water is converted into steam to produce electricity
Ans. (a) Potential energy possessed by stored water is converted into electricity
Explanation: Water is stored behind dams. The stored water has potential energy. When water is released from the dam; Potential energy changes into kinetic energy. This kinetic energy is utilized to turn the turbine to generate electricity.

5. Which is the ultimate source of energy?
(a) Water
(b) Sun
(c) Uranium
(d) Fossil fuels
Ans. (b) Sun
Explanation: Fossil fuels were made from biomass and biomass is made because of photosynthesis. We know that solar energy is converted into biomass through photosynthesis. 
Water cycle is driven by sun and hence kinetic energy in water is because of sun. Hence, it is said that sun is the ultimate source of energy.

6. Which one of the following forms of energy leads to least environmental pollution in the process of its harnessing and utilisation?
(a) Nuclear energy
(b) Thermal energy
(c) Solar energy
(d) Geothermal energy
Ans. (c) Solar energy

7. Ocean thermal energy is due to
(a) energy stored by waves in the ocean
(b) temperature difference at different levels in the ocean
(c) pressure difference at different levels in the ocean
(d) tides arising out in the ocean
Ans. (b) temperature difference at different levels in the ocean
Explanation: Energy from warm surface is used to boil a volatile liquid. Vapours of this liquid are utilized to turn the turbine. Then vapours are channelized to deeper oceans so that they can cool down.

8. The major problem in harnessing nuclear energy is how to
(a) split nuclei?
(b) sustain the reaction?
(c) dispose off spent fuel safely?
(d) convert nuclear energy into electrical energy?
Ans. (c) dispose off spent fuel safely?
Explanation: Spent fuel from a nuclear plant contains highly radioactive material. This can be very harmful for living beings and for the environment.

9. Which part of the solar cooker is responsible for green house effect?
(a) Coating with black colour inside the box
(b) Mirror
(c) Glass sheet
(d) Outer cover of the solar cooker
Ans. (c) Glass sheet

10. The main constituent of biogas is
(a) methane
(b) carbon dioxide
(c) hydrogen
(d) hydrogen sulphide
Ans. (a) Methane

11. The power generated in a windmill
(a) is more in rainy season since damp air would mean more air mass hitting the blades
(b) depends on the height of the tower
(c) depends on wind velocity
(d) can be increased by planting tall trees close to the tower 
Ans. (c) depends on wind velocity
Explanation: Higher wind velocity will turn the turbine at higher speed. This will help in more power generation.

12. Choose the correct statement
(a) Sun can be taken as an inexhaustible source of energy
(b) There is infinite storage of fossil fuel inside the earth
(c) Hydro and wind energy plants are non polluting sources of energy
(d) Waste from a nuclear power plant can be easily disposed off
Ans. (a) Sun can be taken as an inexhaustible source of energy

13. In a hydroelectric power plant more electrical power can be generated if water falls from a greater height because
(a) its temperature increases
(b) larger amount of potential energy is converted into kinetic energy
(c) the electricity content of water increases with height
(d) more water molecules dissociate into ions
Ans. (b) larger amount of potential energy is converted into kinetic energy

Explanation: Potential energy increases with increase in height.

14. Choose the incorrect statement regarding wind power
(a) It is expected to harness wind power to minimum in open space
(b) The potential energy content of wind blowing at high altitudes is the source of wind power
(c) Wind hitting at the blades of a windmill causes them to rotate the rotation thus achieved can be utilised further
(d) One possible method of utilising the energy of rotational motion of the blades of a windmill is to run the turbine of an electric generator
Ans. (b) The potential energy content of wind blowing at high altitudes is the source of wind power
Explanation: A windmill works on kinetic energy of wind.

15. Choose the incorrect statement
(a) We are encouraged to plant more trees so as to ensure clean environment and also provide bio-mass fuel
(b) Gobar-gas is produced when crops, vegetable wastes etc., decompose in the absence of oxygen
(c) The main ingredient of bio-gas is ethane and it gives a lot of smoke and also produces a lot of residual ash
(d) Bio-mass is a renewable source of energy
Ans. (c) The main ingredient of bio-gas is ethane and it gives a lot of smoke and also produces a lot of residual ash
Explanation: The main ingredient of biogas is methane. It burns without smoke and leaves no residue. 
 

Two reasons for the need to harness non-conventional sources of energy are as follows:

(a) Conventional sources are going to be exhausted in near future.

(b) Burning of fossil fuels causes environmental pollution.

Two ways of harnessing energy from ocean are as follows:
 (a) Tidal energy
 (b) Ocean thermal energy

(i) Use of smokeless appliances.
(ii) Afforestation

Plane mirror reflects the sunlight in the solar cooker and thus increases the intensity of solar energy. Glass sheet prevents heat from escaping the solar cooker. In other words, the glass sheet produces greenhouse effect.

(i) Solar cells have no moving parts
(ii) Requires little maintenance
(iii) They can be set up in remote areas

Biomass is a fuel that is obtained from organic materials. Following are some methods of harnessing biomass:
(a) Using firewood as fuel
(b) Using cow dung cakes as fuel
(c) Production of gobar gas

Because of it being erratic, energy cannot be harnessed continuously. Large areas are required for wind farms.

Nuclear energy is harnessed in nuclear power plants. It contains a nuclear reactor, a boiler and a turbine.
• The nuclear reactor is the place where nuclear fission is carried out. Nuclear fission produces a huge amount of energy.
• Energy produced during nuclear fission is utilized to boil water, so that steam can be generated.
• Steam is utilized to turn the turbines to produce electricity.
The following figure shows the working of a nuclear power plant. 

Methods of harnessing solar energy:
Solar Cooker: Solar cooker is a simple device. It is composed of a rectangular box with a mirror at the top. The box has a glass cover. Mirror is kept in erect position so that reflected sunlight can go inside the box. Utensils are kept in the box. Glass cover on top prevents heat from escaping the box.
Solar Cell: A solar cell is also called photovoltaic cell. It converts solar energy into electrical energy. Electrical energy thus produced needs to be stored in inverters so that it can be used even during night.
Solar Furnace: Solar furnace is made of concave mirror. The heating device is kept at the focus of this furnace. Heat from sun is concentrated at the focus which helps in generating lot of heat. 
Solar furnace can be used to heat water and steam generated by this process can be used to generate electricity.
 Limitations of Solar Energy:
• Can be harnessed only at those places which get plenty of sunlight.
• Cannot be harnessed beyond certain latitudes.
• Cannot be harnessed during night.
• Current technologies are very costly.
Ways to overcome limitations: Batteries can be used to store electrical energy from photovoltaic cells. Cheaper technologies need to be discovered.

Conventional Source of Energy	Non Conventional Source of Energy
coal , petroleum, firewood, hydel energy	solar energy, wind energy, nuclear energy, biogas, tidal energy, thermal energy, wave energy

Methods to Harness Tidal Energy:
• For this, a cylindrical structure is vertically placed in coastal areas where tides come.
• A turbine is fitted at the top of this cylinder.
• When the tide comes, air gushes through the cylinder. This turns the turbine and electricity is generated.
• When the tide goes, air moves in opposite direction. This also turns the turbine and electricity is generated.

Following are the main reasons: because of which there is a need to harness non-conventional sources of energy:
 (a) Non-renewable fuels: Fossil fuels are the main energy sources for us. But it takes millions of years for the formation of fossil fuel. The rate at which we are using them means they are not going to last many years from now. They would be definitely exhausted sooner rather than later.
 (b) Polluting fuels: Burning of fossil fuels releases oxides of carbon, nitrogen and Sulphur. These are polluting gases. Carbon dioxide is a greenhouse gas. Carbon monoxide can be poisonous even in low concentration. Oxides of Sulphur and nitrogen cause acid rain. Air pollution also results in increased cases of respiratory problems.
 Following are various ways of harnessing energy from sea:
• Wave energy
• Tidal energy
• Ocean thermal energy 

It is true that energy from various sources have been derived from the sun. Following are
explanations for this:
(a) Plants harness solar energy to make biomass. This biomass was converted into fossil fuels. So, energy in fossil fuels came from sun.
(b) Firewood and gobar gas have got energy from sun; as explained in previous point.
(c) Water cycle on earth happens because of heat from the sun. So, kinetic energy in water is because of sun.
(d) Waves and ocean currents are caused because of heating from the sun. So, different kinds of energy from ocean have sun in their root.
(e) The earth was made after a piece from the sun took shape of this planet. So, geothermal energy or energy from nuclear power plant have come from the sun. 

Biomass is a fuel that is obtained from organic materials.
Principle of Biogas: Biogas is produced by decomposition of organic matter. Biogas is mainly composed of methane. It burns with negligible smoke and leaves no residue. It can be used as kitchen fuel. It can also be used for generating electricity.

Working of Biogas Plant:
• A mixture of farm waste is fed into the biogas plant. This waste is mixed to make a slurry.
• Slurry is then fed into the digester. Digester is an air-tight chamber and oxygen is not present in it.
• Anaerobic bacteria in digester carry out decomposition of slurry. This results in production of biogas.
• Biogas is sent out through an outlet so that it can be suitably used.
• Decomposed matter is taken out and it can be used as manure.